`Effective Faraday rotation´ for broad-spectrum polychromatic light in garnet

For polychromatic light with a broad spectrum width, effective Faraday rotation angle, <θ_F>, is defined operationally, namely as a quantity obtained when measurement is performed in the same was as in the measurement of Faraday rotation, θ_F^λ, for monochromatic light at wavelength λ. Calculation based on superposition theory gave <θ_F> expressed as an expectation value of θ_F^λ weighted by S^λP^λ (or S^λP^λ a ^λ) in the conventional DC (or polarization modulation) method, where S^λ, P^λ and a^λ are the sensitivity of the photodetector, light intensity, and amplitude of polarization modulation, respectively, at wavelength λ. Measurement of <θ_F> on a garnet film of Bi_1.5Y_1.5Fe_4.4Al_0.6O₁₂ around λ=465 nm, where θ_F^λ changes sign, agreed with calculation based on the theory described